Autogenous bone is a composite material composed of impure hydroxyapatite, collagen, and a variety of non-collagenous proteins, as well as embedded and adherent cells. Autogenous hone can be processed into an implantable biomaterial, such as an allograft, for example, by removing the cells, leaving behind the extracellular matrix. The processed bone material can have a variety of properties, depending upon the specific processes and treatments applied to it, and may incorporate characteristics of other biomaterials with which it is combined. For example, bone-derived biomaterials may be processed into load-bearing mineralized grafts that support and integrate with the patient's bone and may alternatively be processed into soft, moldable, or flowable demineralized bone materials that have the ability to induce a cellular healing response.
The use of bone grafts and bone substitute materials in orthopedic medicine is well known. While hone can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a long time to heal, during which the bone is unable to support physiologic loading. Metal pins, screws, and meshes are frequently required to replace the mechanical functions of injured bone. However, metal is significantly stiffer than bone. Use of metal implants may result in decreased bone density around the implantable composite site due to stress shielding. Furthermore, most metal implants are permanent and unable to participate in physiological remodeling.
The inorganic component of human bone is primarily composed of calcium, phosphate ions (Ca2+, PO42−, that form the apatite phase), carbonate ions (CO32−) and small percentages of other ions, such as Mg2+ and Na+, for example.
The carbonate renders the bone-like tissue more “dynamic” (that is, stoichiometrically unstable) and thus more easily reabsorbed by osteoclasts.
One of the most widely used bone-like substitutes in today's surgery is represented by synthetic hydroxyapatite (HA), whose formula will be indicated as Ca10(PO4)6(OH)2. However, this synthetic hydroxyapatite is not a perfect biomimetic substitute of natural bone tissue and is slowly resorbable. In particular, hydroxyapatite alone can remain in a bone defect for long periods preventing resorption. In contrast, pure tricalcium phosphate (TCP) tends to remodel too quickly to provide sufficient scaffolding for new hone ingrowth and can potentially allow for soft tissue to collapse into the bone defect.
Therefore, there exists a need for formulations or implantable composites that remodel more quickly than pure hydroxyapatite, but do not resorb as quickly as pure tricalcium phosphate.